The present invention relates generally to metallurgy, and, more particularly, to a method and apparatus for producing a thixotropic metallic melt through precisely controlled heat transfer and magnetomotive agitation.
The present invention relates in general to an apparatus which is constructed and arranged for producing an xe2x80x9con-demandxe2x80x9d semi-solid material for use in a casting process. Included as part of the overall apparatus are various stations which have the requisite components and structural arrangements which are to be used as part of the process. The method of producing the on-demand semi-solid material, using the disclosed apparatus, is included as part of the present invention.
More specifically, the present invention incorporates electromagnetic stirring and various temperature control and cooling control techniques and apparata to facilitate the production of the semi-solid material within a comparatively short cycle time. Also included are structural arrangements and techniques to discharge the semi-solid material directly into a casting machine shot sleeve. As used herein, the concept of xe2x80x9con-demandxe2x80x9d means that the semi-solid material goes directly to the casting step from the vessel where the material is produced. The semi-solid material is typically referred to as a xe2x80x9cslurryxe2x80x9d and the slug which is produced as a xe2x80x9csingle shotxe2x80x9d is also referred to as a billet.
It is well known that semi-solid metal slurry can be used to produce products with high strength, leak tight and near net shape. However, the viscosity of semi-solid metal is very sensitive to the slurry""s temperature or the corresponding solid fraction. In order to obtain good fluidity at high solid fraction, the primary solid phase of the semi-solid metal should be nearly spherical.
In general, semi-solid processing can be divided into two categories; thixocasting and rheocasting. In thixocasting, the microstructure of the solidifying alloy is modified from dendritic to discrete degenerated dendrite before the alloy is cast into solid feedstock, which will then be re-melted to a semi-solid state and cast into a mold to make the desired part. In rheocasting, liquid metal is cooled to a semi-solid state while its microstructure is modified. The slurry is then formed or cast into a mold to produce the desired part or parts.
The major barrier in rheocasting is the difficulty to generate sufficient slurry within preferred temperature range in a short cycle time. Although the cost of thixocasting is higher due to the additional casting and remelting steps, the implementation of thixocasting in industrial production has far exceeded rheocasting because semi-solid feedstock can be cast in large quantities in separate operations which can be remote in time and space from the reheating and forming steps.
In a semi-solid casting process, generally, a slurry is formed during solidification consisting of dendritic solid particles whose form is preserved. Initially, dendritic particles nucleate and grow as equiaxed dendrites within the molten alloy in the early stages of slurry or semi-solid formation. With the appropriate cooling rate and stirring, the dendritic particle branches grow larger and the dendrite arms have time to coarsen so that the primary and secondary dendrite arm spacing increases. During this growth stage in the presence of stirring, the dendrite arms come into contact and become fragmented to form degenerate dendritic particles. At the holding temperature, the particles continue to coarsen and become more rounded and approach an ideal spherical shape. The extent of rounding is controlled by the holding time selected for the process. With stirring, the point of xe2x80x9ccoherencyxe2x80x9d (the dendrites become a tangled structure) is not reached. The semi-solid material comprised of fragmented, degenerate dendrite particles continues to deform at low shear force.
When the desired fraction solid and particle size and shape have been attained, the semi-solid material is ready to be formed by injecting into a die-mold or some other forming process. Solid phase particle size is controlled in the process by limiting the slurry creation process to temperatures above the point at which the solid phase begins to form and particle coarsening begins.
It is known that the dendritic structure of the primary solid of a semi-solid alloy can be modified to become nearly spherical by introducing the following perturbation in the liquid alloy near liquidus temperature or semi-solid alloy:
1) Stirring: mechanical stirring or electromagnetic stirring;
2) Agitation: low frequency vibration, high-frequency wave, electric shock, or electromagnetic wave;
3) Equiaxed Nucleation: rapid under-cooling, grain refiner;
4) Oswald Ripening and Coarsening: holding alloy in semi-solid temperature for a long time.
While the methods in (2)-(4) have been proven effective in modifying the microstructure of semi-solid alloy, they have the common limitation of not being efficient in the processing of a high volume of alloy with a short preparation time due to the following characteristics or requirements of semi-solid metals:
High dampening effect in vibration.
Small penetration depth for electromagnetic waves.
High latent heat against rapid under-cooling.
Additional cost and recycling problem to add grain refiners.
Natural ripening takes a long time, precluding a short cycle time.
While most of the prior art developments have been focused on the microstructure and rheology of semi-solid alloy, temperature control has been found by the present inventors to be one of the most critical parameters for reliable and efficient semi-solid processing with a comparatively short cycle time. As the apparent viscosity of semi-solid metal increases exponentially with the solid fraction, a small temperature difference in the alloy with 40% or higher solid fraction results in significant changes in its fluidity. In fact, the greatest barrier in using methods (2)-(4), as listed above, to produce semi-solid metal is the lack of stirring. Without stirring, it is very difficult to make alloy slurry with the required uniform temperature and microstructure, especially when the there is a requirement for a high volume of the alloy. Without stirring, the only way to heat/cool semi-solid metal without creating a large temperature difference is to use a slow heating/cooling process. Such a process often requires that multiple billets of feedstock be processed simultaneously under a pre-programmed furnace and conveyor system, which is expensive, hard to maintain, and difficult to control.
While using high-speed mechanical stirring within an annular thin gap can generate high shear rate sufficient to break up the dendrites in a semi-solid metal mixture, the thin gap becomes a limit to the process""s volumetric throughput. The combination of high temperature, high corrosion (e.g. of molten aluminum alloy) and high wearing of semi-solid slurry also makes it very difficult to design, to select the proper materials and to maintain the stirring mechanism.
Prior references disclose the process of forming a semi-solid slurry by reheating a solid billet forming by thixocasting or by directly from the melt using mechanical or electromagnetic stirring. The known methods for producing semi-solid alloy slurries include mechanical stirring and inductive electromagnetic stirring. The processes for forming a slurry with the desired structure are controlled, in part, by the interactive influences of the shear and solidification rates.
In the early 1980""s, an electromagnetic stirring process was developed to cast semi-solid feedstock with discrete degenerate dendrites. The feedstock is cut to proper size and then remelt to semi-solid state before being injected into mold cavity. Although this magneto hydrodynamic (MHD) casting process is capable of generating high volume of semi-solid feedstock with adequate discrete degenerate dendrites, the material handling cost to cast a billet and to remelt it back to a semi-solid composition reduces the competitiveness of this semi-solid process compared to other casting processes, e.g. gravity casting, low-pressure die-casting or high-pressure die-casting. Most of all, the complexity of billet heating equipment, the slow billet heating process and the difficulties in billet temperature control have been the major technical barriers in semi-solid forming of this type.
The billet reheating process provides a slurry or semi-solid material for the production of semi-solid formed (SSF) products. While this process has been used extensively, there is a limited range of castable alloys. Further, a high fraction of solids (0.7 to 0.8) is required to provide for the mechanical strength required in processing with this form of feedstock. Cost has been another major limitation of this approach due to the required processes of billet casting, handling, and reheating as compared to the direct application of a molten metal feedstock in the competitive die and squeeze casting processes.
In the mechanical stirring process to form a slurry or semi-solid material, the attack on the rotor by reactive metals results in corrosion products that contaminate the solidifying metal. Furthermore, the annulus formed between the outer edge of the rotor blades and the inner vessel wall within the mixing vessel results in a low shear zone while shear band formation may occur in the transition zone between the high and low shear rate zones. There have been a number of electromagnetic stirring methods described and used in preparing slurry for thixocasting billets for the SSF process, but little mention has been made of an application for rheocasting.
The rheocasting, i.e., the production by stirring of a liquid metal to form semi-solid slurry that would immediately be shaped, has not been industrialized so far. It is clear that rheocasting should overcome most of limitations of thixocasting. However, in order to become an industrial production technology, i.e., producing stable, deliverable semi-solid slurry on-line (i.e., on-demand) rheocasting must overcome the following practical challenges: cooling rate control, microstructure control, uniformity of temperature and microstructure, the large volume and size of slurry, short cycle time control and the handling of different types of alloys, as well as the means and method of transferring the slurry to a vessel and directly from the vessel to the casting shot sleeve.
While propeller-type mechanical stirring has been used in the context of making a semi-solid slurry, there are certain problems or limitations. For example, the high temperature and the corrosive and high wearing characteristics of semi-solid slurry make it very difficult to design a reliable slurry apparatus with mechanical stirring. However, the most critical limitation of using mechanical stirring in rheocasting is that its small throughput cannot meet the requirements of production capacity. It is also known that semi-solid metal with discrete degenerated dendrite can also be made by introducing low frequency mechanical vibration, high-frequency ultra-sonic waves, or electric-magnetic agitation with a solenoid coil. While these processes may work for smaller samples at slower cycle time, they are not effective in making larger billet because of the limitation in penetration depth. Another type of process is solenoidal induction agitation, because of its limited magnetic field penetration depth and unnecessary heat generation, it has many technological problems to implement for productivity. Vigorous electromagnetic stirring is the most widely used industrial process permits the production of a large volume of slurry. Importantly, this is applicable to any high-temperature alloys.
Two main variants of vigorous electromagnetic stirring exist, one is rotational stator stirring, and the other is linear stator stirring. With rotational stator stirring, the molten metal is moving in a quasiisothermal plane, therefore, the degeneration of dendrites is achieved by dominant mechanical shear. U.S. Pat. No. 4,434,837, issued Mar. 6, 1984 to Winter et al., describes an electromagnetic stirring apparatus for the continuous making of thixotropic metal slurries in which a stator having a single two pole arrangement generates a non-zero rotating magnetic field which moves transversely of a longitudinal axis. The moving magnetic field provides a magnetic stirring force directed tangentially to the metal container, which produces a shear rate of at least 50 secxe2x88x921 to break down the dendrites. With linear stator stirring, the slurries within the mesh zone are re-circulated to the higher temperature zone and remelted, therefore, the thermal processes play a more important role in breaking down the dendrites. U.S. Pat. No. 5,219,018, issued Jun. 15, 1993 to Meyer, describes a method of producing thixotropic metallic products by continuous casting with polyphase current electromagnetic agitation. This method achieves the conversion of the dendrites into nodules by causing a refusion of the surface of these dendrites by a continuous transfer of the cold zone where they form towards a hotter zone.
It is known in the art that thixotropic metal melts may be produced by agitating a cooling metal melt. As the metal melt approaches its liquidus temperature, a particulate sold phase begins to precipitate out. As the melt cools, the amount of solid phase increases relative to the remaining liquid phase. Also, the composition at the liquid phase may vary as a function of its the ratio of the amount of remaining liquid phase to the total amount of solid and liquid phases. The viscosity of the cooling melt is sensitive to its temperature, its solid-to-liquid ratio, the composition of the remaining liquid phase, and the relative size, number, and shape of the solid particles. In particular, if the forming solid particles are irregular, the viscosity of the forming semi-solid slurry tends to be substantially greater than if the particles are spherical or spheroid. The viscosity of the semi-solid slurry is even greater if the forming metallic particles are dendritic.
It is well known that a semi-solid metallic slurry may be produced having substantially regularly shaped particles by agitating the cooling melt to degenerate the forming dendrites. Known agitation techniques include mechanical stirring, vibration, induction agitation, undercooling, and high-voltage electric pulse injection. However, these techniques do not address the issue of maintaining the slurry at a uniform, equilibrated temperature. If temperature differentials exist within the melt, the distribution and growth of the solid particulate phase will be irregular and the viscosity of the slurry will likewise be non-uniform. Moreover, temperature differentials in the slurry increase the likelihood of the onset of cascade crystallization of all or part of the slurry. This is especially true with regard to the formation of a solid metallic skin around the slurry, since heat extraction from the slurry occurs primarily at the container-slurry interface.
Another disadvantage with the known techniques and apparata for producing semi-solid slurries is that they are ill suited for continuous or large-scale processing. In addition to the above-described disadvantages, the prior art techniques take on the order of 6-8 minutes to process a molten metal charge into a thixotropic slurry ready for molding. Moreover, the known techniques necessitate a step for transferring molten metal from a melting furnace into a separate stirring vessel, exposing the molten metal to ambient gasses and increasing the possibility of reaction contaminants forming in the liquid metal.
There is therefore a need for a system capable of both quickly and efficiently producing molten metal charge and of mixing the melt to produce a thermally equilibrated thixotropic metal slurry ready for molding from the molten metal charge under a controlled atmosphere. The present invention addresses this need in a novel and unobvious manner.
The present invention relates to a method and apparatus for producing a thixotropic metallic melt by simultaneously controlledly cooling and stirring the melt such that solid particles of a first phase begin to precipitate in a residual liquid second phase. Dendritic growth of the solid particles is curtailed by vigorously stirring the metallic melt, resulting in degenerate dendritic particles having a substantially spheroidal character. The metallic melt is stirred such that the metal is rapidly and efficiently circulated, so as to quickly reach a substantially uniform temperature throughout. Through precision stirring and cooling, the metallic melt is maintained with about 70-80% of the melt being solid spheroidal particles of a first phase suspended in a liquid medium of a second phase.
One form of the present invention is an apparatus for forming a molten metal mass from solid metal processors under an inert gas atmosphere, automatically transferring a portion of the molten metal mass into a mixing chamber, and rapidly cooling and stirring the transferred portion of molten metal to form a thixotropic semi-solid metallic slurry suitable for molding.
One object of the present invention is to provide an improved system for the production of a thixotropic metallic melt comprising a first phase of degenerate dendritic solid particles suspended in a second liquid phase, wherein the first phase comprises about 70-80 percent of the melt. Related objects and advantages of the present invention will be apparent from the following description.